On a foggy Monday in 1986, the universe suddenly, without warning, bifurcated.

Fast-forward to 35 years later: Felix Sayers is a culinary writer living in San Francisco of Universe A who spends his days lunching at Coconut Café and dreaming of penning an Agatha Christie-style mystery. But everything changes when his Aunt Henrietta dies, leaving Felix a photograph of his father and himself--dated ten days before Felix was born. It can only mean one thing: Felix has an 'alter' in Universe B. Panicked that his mystery novel may exist already, Felix crosses to San Francisco B and proceeds to flagrantly violate the rules of both worlds by snooping around his alter's life. But when he narrowly escapes a hit-and-run, it becomes clear that someone knows he's crossed over... and whoever it is isn't happy about it. Now Felix must uncover the truth about his alter, the events of one Monday, and a wayward rubber duck before his time in both worlds runs out.